SCR NOx Pre-Catalyst Efficiency Below Threshold Bank 1

Check these first

• A degraded or contaminated SCR catalyst
• A failed or slow upstream NOx sensor
• Poor quality or contaminated diesel exhaust fluid
• An exhaust leak ahead of the SCR catalyst
• A reductant dosing injector that is not spraying correctly

What to do next

1. Retrieve all NOx, SCR and DEF related codes before testing
2. Inspect the exhaust ahead of the catalyst for leaks and soot tracking
3. Check diesel exhaust fluid level, age and quality
4. Compare upstream and downstream NOx sensor readings with a scan tool
5. Replace the NOx sensor or SCR catalyst once the circuit tests good
6. Clear the code and retest after a full regeneration drive cycle

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